A known radio frequency passive acoustic transponder system produces individualized responses to an interrogation signal. The code space for these devices may be, for example, 2.sup.16 codes, or more, allowing a large number of transponders to be produced without code reuse. These devices provide a piezoelectric substrate on which an aluminum pattern is formed, for example by a typical microphotolithography process, with a minimum feature size of, for example, one micron.
These codes are imposed upon a received signal by signal transforming elements formed on the substrate. Typically, for each encoded symbol, a separate transforming element is formed. In order to facilitate decoding, the transforming elements interact with the interrogation signal at different respective time delays, which are ensured by the acoustic propagation delay on the substrate. These time delays therefore dictate the minimum path length, and therefore size of the substrate. As in many microphotolithographic systems, the substrate size is related to system cost, and smaller substrates imply lower costs. Therefore, a tradeoff exists between substrate size and available encoding complexity.
The aforementioned transponder devices include a surface acoustic wave device, in which an identification code is provided as a characteristic time-domain reflection, attenuation, phase delay, and/or transducer interaction pattern in a retransmitted signal, in a system which generally requires that the signal emitted from an exciting antenna be non-stationary with respect to a signal received from the tag. This ensures that the reflected signal pattern is distinguished from the emitted signal, and can be analyzed in a plurality of states. This analysis reveals the various delay components within the device. In such a device, received RF energy, possibly with harmonic conversion, is emitted onto a piezoelectric substrate as an acoustic wave by means of an interdigital electrode system, from whence it travels through the substrate, interacting with reflecting, delay or resonant/frequency selective elements in the path of the wave. A portion of the acoustic wave is ultimately received by an interdigital electrode system, which may be the same or different than the launch transducer, and retransmitted. These devices do not require a semiconductor memory nor external electrical energy storage system, e.g., battery or capacitor, to operate. The propagation velocity of an acoustic wave in a surface acoustic wave device is slow as compared to the free space propagation velocity of a radio wave. Thus, the time for transmission between the radio frequency interrogation system and the transponder is typically short as compared to the acoustic delay of the substrate. This allows the rate of the interrogation frequency change to be based primarily on the delay characteristics within the transponder, without requiring measurements of the distance between the transponder and the interrogation system antenna.
The interrogation frequency is controlled to change sufficiently from the return or "backscatter" signal from the transponder, so that a return signal having a minimum delay may be distinguished from the interrogation frequency, and so that all of the relevant delays are unambiguously received for analysis. The interrogation frequency thus should not return to the same frequency within a minimum time-period. Generally, such systems are interrogated with a pulse transmitter or chirp frequency system.
Systems for interrogating a passive transponder employing acoustic wave devices, carrying amplitude and/or phase-encoded information are disclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein by reference. Other passive interrogator label systems are disclosed in the U.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217, expressly incorporated herein by reference.
Passive transponder tag interrogation systems are also known with separate receiving and transmitting antennas, which may be at the same frequency or harmonically related, and having the same or different polarization. Thus, in these systems, the transmitted and received signals may be distinguished other than by frequency. The acoustic wave is often a surface acoustic wave, although acoustic wave devices operating with various other wave types, such as bulk waves, are known.
The information code associated with and which identifies the passive transponder is built into the transponder at the time that a layer of metallization is finally defined on the substrate of piezoelectric material. This metallization also defines the antenna coupling, launch transducers, acoustic pathways and information code elements, e.g., reflectors and delay elements. Thus, the information code in this case is non-volatile and permanent. The information representing these elements is present in the return signal as a set of characteristic perturbations of the interrogation signal, such as a specific complex delay pattern and attenuation characteristics. In the case of a transponder tag having launch transducers and a variable pattern of reflective elements, the number of possible codes is N.times..sub.2.sup.M where N is the number of acoustic waves launched by the transducers and M is the number of reflective element positions for each transducer. Thus, with four launch transducers each emitting two acoustic waves, and a potential set of eight variable reflective elements in each acoustic path, the number of differently coded transducers is 2048. Therefore, for a large number of potential codes, it is necessary to provide a large number of launch transducers and/or a large number of reflective elements. However, efficiency is lost with increasing acoustic path complexity (i.e., power splitting), and a large number of distinct acoustic waves reduces the signal strength of the signal encoding the information in each. Therefore, the transponder design is a tradeoff between device codespace complexity and efficiency.
The known passive acoustic transponder tag thus typically includes a multiplicity of "signal conditioning elements", i.e., delay elements, reflectors, and/or amplitude modulators, which are coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the acoustic wave interacting with it. Even where the signal is split into multiple portions, it is advantageous to reradiate the signal through a single antenna. Therefore, a "signal combining element" coupled to the all of the acoustic waves, which have interacted with the signal conditioning elements, is provided for combining the intermediate signals to produce the radiated transponder signal. The radiated signal is thus a complex composite of all of the signal modifications, which may occur within the transponder, modulated on the interrogation wave.
In known passive acoustic transponder systems, the transponder remains static over time, so that the encoded information is retrieved by a single interrogation cycle, representing the state of the tag, or more typically, obtained as an inherent signature of an emitted signal due to internal time delays. In order to determine a transfer function of a passive transponder device, the interrogation cycle may include measurements of excitation of the transponder at a number of different frequencies. This technique allows a frequency domain analysis, rather than a time domain analysis of an impulse response of the transponder. This is particularly important since time domain analysis requires very high time domain resolution, e.g., &lt;100 nS, to accurately capture the characteristics of the encoding, while frequency domain analysis does not impose such stringent requirements on the analysis system.
Passive transponder encoding schemes include selective modification of interrogation signal transfer function H(s) and delay functions f(z). These functions therefore typically generate a return signal in the same band as the interrogation signal. Since the return signal is mixed with the interrogation signal, the difference between the two will generally define the information signal, along with possible interference and noise. By controlling the rate of change of the interrogation signal frequency with respect to a maximum round trip propagation delay, including internal delay, as well as possible Doppler shift, the maximum bandwidth of the demodulated signal may be controlled. Thus, the known systems seek to employ a chirp interrogation waveform which allows a relatively simple processing of limited bandwidth signals.
Typically, the interrogator transmits a first signal having a first frequency that successively assumes a plurality of incremental frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905-925 MHz, referred to herein as the nominal 915 MHz band, a frequency band that is commonly available for unlicensed use. Of course, other bands may be used, and preferably these are bands which do not require a license, and are available worldwide for use. These bands extend, for example, from 100 MHz to 25 GHz. Of course, licensed bands and locally available bands may be used. The response of the tag to excitation at any given frequency is distinguishable from the response at other frequencies, due to the relationship of the particular frequency and fixed time delays.
Preferably, the passive acoustic wave transponder tag includes at least one element having predetermined characteristics, which assists in synchronizing the receiver and allowing for temperature compensation of the system. As the temperature changes, the piezoelectric substrate may expand and contract, altering the characteristic delays and other parameters of the tag. Variations in the transponder response due to changes in temperature thus result, in part, from the thermal expansion of the substrate material. Although propagation distances are small, an increase in temperature of only 20.degree. C. can produce an increase in propagation time by the period of one entire cycle at a transponder frequency of about 915 MHz; correspondingly, a change of about 1.degree. C. results in a relative phase change of about 18.degree..
A known transponder is constructed such that i.sup.th delay time t.sub.i =T.sub.0 +K.DELTA.T+.DELTA.V.sub.i, where K is a proportionality constant, .DELTA.T is the nominal, known difference in delay time between the intermediate signals of two particular successive ones of the signal delay elements in the group, and .quadrature.V.sub.i is a modification factor due to inter-transponder variations, such as manufacturing variations. By measuring the quantities .DELTA.T and .DELTA.V.sub.i, it is possible, according to known techniques, to determine the expected delay time t.sub.i -T.sub.0 for each and every signal delay element from the known quantities K, .DELTA.T and .DELTA.V.sub.i. The manufacturing variations n.DELTA.V.sub.i comprise a "mask" variation .DELTA.M.sub.i due to imperfections in the photolithographic mask; an "offset" variation .DELTA.O.sub.i which arises from the manufacturing process used to deposit the metal layer on the piezoelectric substrate; and a random variation .DELTA.R.sub.i which is completely unpredictable but usually neglectably small. Specific techniques are available for determining and compensating both the mask variations .DELTA.M.sub.i and the offset variations .DELTA.O.sub.i.
The known chirp interrogation system for interrogating surface acoustic wave transponder system provides a number of advantages, including high signal-to-noise performance. Further, the output of the signal mixer--namely, the signal which contains the instantaneous difference frequencies of the interrogating chirp signal and the transponder reply signal, typically fall in (or may be made to fall in) the range below 3000 Hz, and thus may be transmitted over inexpensive, shielded, audio-grade twisted-pair wires, and indeed may possibly be transmitted over the telephony infrastructure. Furthermore, since signals of this type are not greatly attenuated or dispersed when transmitted over long distances, the signal processor may be located at a position quite remote from the signal mixer, or provided as a central processing site for multiple interrogator antennae.
Another known type of interrogation system employs impulse excitation. These systems require broadband transponder signal analysis, and thus cannot typically employ audio frequency (low frequency) analysis systems. This impulse excitation interrogation system does not seek to analyze the response of fixed elements within the passive transponder to a plurality of different excitation challenges.
A known surface acoustic wave passive interrogator label system, as described, for example, in U.S. Pat. Nos. 4,734,698; 4,737,790; 4,703,327; and 4,951,057, expressly incorporated herein by reference, includes an interrogator having a voltage controlled oscillator which produces a first radio frequency signal determined by a control voltage. This first signal is amplified by a power amplifier and applied to an antenna for transmission to a remote transponder as an interrogation signal. As is known, the voltage controlled oscillator may be replaced with other oscillator (or signal generator) types.
The first signal is received by an antenna of the remote transponder and passed to a signal transforming element, which converts the first (interrogation) signal into a second (reply) signal, encoded with a characterizing information pattern. The information pattern is encoded by a series of elements having characteristic delay periods T.sub.0 and .DELTA.T.sub.1, .DELTA.T.sub.2, . . . .DELTA.T.sub.N.
Two common types of systems exist. In a first, the delay periods correspond to physical delays in the propagation of the acoustic signal. After passing each successive delay, a portion of the signal I.sub.0, I.sub.1, I.sub.2 . . . I.sub.N is tapped off and supplied to a summing element. The resulting signal, which is the sum of the intermediate signals I.sub.0 . . . I.sub.N, is fed back to a transponder tag antenna, which may be the same or different than the antenna which received the interrogation signal, for transmission to the interrogator/receiver antenna. In a second system, the return signal is composed of sets of reflected signals, resulting from reflectors in the path of the signal which reflect portions of the acoustic wave back to the launch transducer, where they are converted back to an electrical signal and emitted by the transponder tag antenna. The second signal is passed either to the same or different antenna of the remote transponder for transmission back to the interrogator/receiver apparatus. In both cases, between the taps or reflectors, signal modification elements, such as delay pads, selectively modify the signal. This second signal carries encoded information which, at a minimum, identifies the particular transponder.
The transponder serves as a signal transforming element, which comprises N+1 signal conditioning elements and a signal combining element, where necessary. The signal conditioning elements are selectively provided to impart a different response code for different transponders, and which may involve separate intermediate signals I.sub.0, I.sub.1 . . . I.sub.N within the transponder. Each signal conditioning element comprises a known delay T.sub.i and a known amplitude modification A.sub.i (either attenuation or amplification). The respective delay T.sub.i and amplitude modification A.sub.i may be functions of the frequency of the received first signal, may provide a constant delay and constant amplitude modification, respectively, independent of frequency or may have differing dependency on frequency. The order of the delay and amplitude modification elements may be reversed; that is, the amplitude modification elements A.sub.i may precede the delay elements T.sub.i. Amplitude modification A.sub.i can also occur within the path T.sub.i. The signals are combined, where necessary, in a combining element which combines these intermediate signals (e.g., by addition, multiplication or the like) to form the second (reply) signal S2 and the combined signal is emitted by the transponder antenna.
The second signal is picked up by a receiving antenna of the interrogation apparatus. Both this second signal and the first signal (or respective signals derived from these two signals) are applied to a mixer (four quadrant multiplier) to produce a third signal containing frequencies which include both the sums and the differences of the frequencies contained in the first and second signals. The third signal is then low-pass filtered (to pass the difference frequency and block the input signals and sum frequency), digitized and passed to a digital signal processor which determines the amplitude a.sub.i and the respective phase .phi..sub.i of each frequency component f.sub.i among a set of frequency components (f.sub.0, f.sub.1, f.sub.2 . . . ) in the filtered third signal. The filter thus distinguishes the sum and difference components, and prevents aliasing in the analog-to-digital converter. Typically, the low pass filter is set to have a narrow passband, to filter transients and reduce Gaussian noise. For example, in a known system with a frequency hopping rate of 8,000 per second, the filter has a cutoff of about 3,000 Hz. This narrow bandwidth allows a relatively slow analog to digital converter, e.g., about 10 ksps, to be employed to digitize the signal.
Each phase .phi..sub.i is determined with respect to the phase .phi..sub.0 =0 of the lowest frequency component f.sub.0. The third signal may be intermittently supplied to the mixer by means of a switch, and indeed the signal processor may be time-division multiplexed to handle a plurality of mixed (demodulated) signals from different antennas.
The information determined by the digital signal processor is passed to a microprocessor computer system. This computer system analyzes the frequency, amplitude and phase information and makes decisions based upon this information. For example, the computer system may determine the identification number of the interrogated transponder. This identification number and/or other decoded information is made available at an output.
In one known interrogation system embodiment, the voltage controlled oscillator is controlled to produce a sinusoidal RF signal with a frequency that is incrementally swept in 128 equal discrete steps from 905 MHz to 925 MHz. Each frequency step is maintained for a period of 125 microseconds so that the entire frequency sweep is carried out in 16 milliseconds, with a step rate of 8 kHz. Thereafter, the frequency is dropped back to 905 MHz in a relaxation period of 0.67 milliseconds. This stepwise frequency sweep approximates a linear frequency sweep. In this embodiment, each delayed component within the reply (second) signal has a different frequency with respect to the instantaneous interrogation (first) signal.
Assuming a round-trip, radiation transmission time of t.sub.0, the total round-trip times between the moment of transmission of the first signal and the moments of reply of the second signal will be t.sub.0 +T.sub.0, t.sub.0 +T.sub.1, . . . t.sub.0 +T.sub.N, for the delays T.sub.0N, T . . . , T.sub.1 respectively. Considering only the transponder delay T.sub.N, at the time t.sub.R when the second (reply) signal is received at the antenna, the frequency of this second signal will be .DELTA.f.sub.N less than the instantaneous frequency of the first signal transmitted by the interrogator system antenna. Thus, if the first and second signals are mixed or "homodyned", this frequency difference .DELTA.f.sub.N will appear in the third signal as a beat frequency. Understandably, other beat frequencies will also result from the other delayed frequency spectra resulting from the time delays T.sub.0, T.sub.1 . . . . T.sub.N-1. In the case of a "chirp" waveform, the difference between the emitted and received waveform will generally be constant, and therefore the relationship of each delayed component can be determined.
As can be seen, in this embodiment, all significant components of the third (mixed) signal will be within a limited range defined by the maximum delay within the transponder signal transformer and the chirp rate. Thus, this signal may be band limited within this range without loss of significant information. In a known system with a chirp range of 20 MHz, over a cycle period of 16 mS, with 128 transitions, the frequency difference per transition is 156,250 Hz.
In one embodiment of a passive transponder, the internal circuit is a surface acoustic wave device which operates to convert the received first signal into an acoustic wave, and then to reconvert the acoustic energy back into the second signal for transmission via a dipole antenna. The signal transforming element of the transponder includes a piezoelectric substrate material such as a lithium niobate (LiNbO.sub.3) crystal, which has a free surface acoustic wave propagation velocity of about 3488 meters/second. The substrate is, for example, a 3 mm by 5 mm rectangular slab having a thickness of 0.5 mm. On the surface of this substrate is deposited a layer of metal, such as aluminum, forming a pattern which includes transducers and delay/reflective elements. Each delay element has a width sufficient to delay the propagation of the surface acoustic wave from one tap transducer to the next by one quarter cycle or 90.degree. with respect to an undelayed wave at the frequency of operation (in the 915 MHz band). By providing locations for three delay pads between successive tap transducers, the phase.phi. of the surface acoustic wave received by a tap transducer may be controlled to provide four phase possibilities, zero pads=0.degree.; one pad=90.degree.; two pads=180.degree.; and three pads=270.degree.. These pads may be selectively deposited as a metallization layer during manufacture, or formed in a complete complement and selectively removed during a secondary process to encode the transponder. Where a reflective element returns the signal to the launch transducer, the delays are calculated based on two passes over the pad. Typically, a reflective or semireflective element is provided between each set of delay pads to allow them to be distinguished, and allowing, in the case of semireflective elements, for a series of sets of delay pads to be disposed along the path of an acoustic wave. As the number of sets of delay pads increases, the signal to noise ratio in the transponder reply signal is severely degraded. This limitation on the number of tap transducers places a limitation on the length of the informational code imparted in the transponder replies.
A plurality of launch transducers may be connected to common bus bars which, in turn, are connected to the dipole antenna of the transponder. Each launch transducer may have a forward and backward wave, and, indeed, care must be taken to damp a reverse wave where this emission is undesired in order to reduce interference. Thus, the codespace of the transponder may include a plurality of sets of encoding elements, each associated with a particular wavepath. Opposite each launch transducer is one or more reflectors, which reflect surface acoustic waves back toward the transducers which launched them. Since the transducers are connected in parallel, a radio frequency interrogation pulse is received by all the transducers essentially simultaneously. Consequently, these transducers simultaneously generate surface acoustic waves which are transmitted outward in both directions. The system is configured so that the reflected surface acoustic waves are received by their respective transducers at staggered intervals, so that a single interrogation pulse produces a series of reply pulses after respective periods of delay.
FIG. 1 shows a known an interrogator system comprising a voltage controlled oscillator 10 which produces a first signal S1 at a radio frequency determined by a control voltage V supplied by a control unit 12. This signal S1 is amplified by a power amplifier 14 and applied to an antenna 16 for transmission to a transponder 20.
The signal S1 is received at the antenna 18 of the transponder 20 and passed to a signal transforming element 22. This signal transformer converts the first (interrogation) signal S1 into a second (reply) signal S2, encoded with an information pattern. The information pattern is encoded as a series of elements having characteristic delay periods T.sub.0 and .DELTA.T.sub.1, .DELTA.T.sub.2, . . . .DELTA.T.sub.N. Two common types of transducer devices exist. In a first, shown schematically in FIG. 5, the delay periods correspond to physical delays in the propagation of the acoustic signal. After passing each successive delay, a portion of the signal I.sub.0, I.sub.1, I.sub.2 . . . I.sub.N is tapped off and supplied to a summing element. The resulting signal S2, which is the sum of the intermediate signals I.sub.0 . . . I.sub.N, is fed back to a transponder tag antenna, which may be the same or different than the antenna which received the interrogation signal, for transmission to the interrogator/receiver antenna. In a second system, shown schematically in FIG. 4, the delay periods correspond to the positions of reflective elements, which reflect portions of the acoustic wave back to the launch transducer, where they are converted back to an electrical signal and emitted by the transponder tag antenna.
The signal S2 is passed either to the same antenna 18 or to a different antenna 24 for transmission back to the interrogator/receiver apparatus. This second signal S2 carries encoded information which, at a minimum, identifies the particular transponder 20.
The signal S2 is picked up by a receiving antenna 26. Both this second signal S2 and the first signal S1 (or respective signals derived from these two signals) are applied to a mixer (four quadrant multiplier) 30 to produce a third signal S3 containing frequencies which include both the sums and the differences of the frequencies contained in the signals S1 and S2. The signal S3 is passed to a signal processor 32 which determines the amplitude a.sub.i and the respective phase .phi..sub.i of each frequency component f.sub.i among a set of frequency components (f.sub.0, f.sub.1, f.sub.2 . . . ) in the signal S3. Each phase .phi..sub.i is determined with respect to the phase .phi..sub.0 =0 of the lowest frequency component f.sub.0. The signal S3 may be intermittently supplied to the mixer by means of a switch.
The information determined by the signal processor 32 is passed to a computer system comprising, among other elements, a random access memory (RAM) 34 and a microprocessor 36. This computer system analyzes the frequency, amplitude and phase information and makes decisions based upon this information. For example, the computer system may determine the identification number of the interrogated transponder 20. This I.D number and/or other decoded information is made available at an output 38.
The transponder, as shown in FIG. 2, serves as a signal transforming element 22, which comprises N+1 signal conditioning elements 40 and a signal combining element 42. The signal conditioning elements 40 are selectively provided to impart a different response code for different transponders, and which may involve separate intermediate signals I.sub.0, I.sub.1, . . . I.sub.N within the transponder. Each signal conditioning element 40 comprises a known delay T.sub.i and a known amplitude modification A.sub.i (either attenuation or amplification). The respective delay T.sub.i and amplitude modification A.sub.i may be functions of the frequency of the received signal S1, or they may provide a constant delay and constant amplitude modification, respectively, independent of frequency. The time delay and amplitude modification may also have differing dependency on frequency. The order of the delay and amplitude modification elements may be reversed; that is, the amplitude modification elements A.sub.i may precede the delay elements T.sub.i. Amplitude modification A.sub.i can also occur within the path T.sub.i.
The signals are combined in combining element 42 which combines these intermediate signals (e.g., by addition, multiplication or the like) to form the reply signal S2 and the combined signal emitted by the antenna 18.
In one embodiment, as shown in FIG. 3A and 3B, the voltage controlled oscillator 10 is controlled to produce a sinusoidal RF signal with a frequency that is swept in 128 equal discrete steps from 905 MHz to 925 MHz. Each frequency step is maintained for a period of 125 microseconds so that the entire frequency sweep is carried out in 16 milliseconds. Thereafter, the frequency is dropped back to 905 MHz in a relaxation period of 0.67 milliseconds. The stepwise frequency sweep 46 shown in FIG. 3B thus approximates the linear sweep 44 shown in FIG. 3A.
Assuming that the stepwise frequency sweep 44 approximates an average, linear frequency sweep or "chirp" 47, FIG. 3B illustrates how the transponder 20, with its known, discrete time delays T.sub.0, T.sub.1 . . . T.sub.N produces the second (reply) signal 52 with distinct differences in frequency from the first (interrogation) signal 51. Assuming a round-trip, radiation transmission time of t.sub.0, the total round-trip times between the moment of transmission of the first signal and the moments of reply of the second signal will be t.sub.0 +T.sub.0, t.sub.0 +T.sub.1, . . . t.sub.0 +T.sub.N, for the delays T.sub.0N, T . . . , T.sub.1 respectively. Considering only the transponder delay T.sub.N, at the time t.sub.R when the second (reply) signal is received at the antenna 26, the frequency 48 of this second signal will be .DELTA.f.sub.N less than the instantaneous frequency 47 of the first signal S1 transmitted by the antenna 16. Thus, if the first and second signals are mixed or "homodyned", this frequency difference .DELTA.f.sub.N will appear in the third signal as a beat frequency. Understandably, other beat frequencies will also result from the other delayed frequency spectra 49 resulting from the time delays T.sub.0, T.sub.1 . . . T.sub.N-1. Thus, in the case of a "chirp" waveform, the difference between the emitted and received waveform will generally be constant.
In mathematical terms, we assume that the phase of a transmitted interrogation signal is .phi.=2.pi.f.tau., where .tau. is the round-trip transmission time delay. For a ramped frequency df/dt or f, we have: 2.pi.f.tau.=d.phi./dt=.omega.. .omega., the beat frequency, is thus determined by T for a given ramped frequency or chirp f. In this case, the (mixed) signal S3 may be analyzed by determining a frequency content of the S3 signal, for example by applying it to sixteen bandpass filters, each tuned to a different frequency, f.sub.0, f.sub.1, . . . f.sub.E, f.sub.F. The signal processor determines the amplitude and phase of the signals that pass through these respective filters. These amplitudes and phases contain the code or "signature" of the particular signal transformer 22 of the interrogated transponder 20. This signature may be analyzed and decoded in known manner.
In one embodiment of a passive transponder, shown in FIGS. 6 and 7, the internal circuit operates to convert the received signal S1 into an acoustic wave and then to reconvert the acoustic energy back into an electrical signal S2 for transmission via a dipole antenna 70, connected t.sub.0, and arranged adjacent a SAW device made of a substrate 72. More particularly, the signal transforming element of the transponder includes a substrate 72 of piezoelectric material such as a lithium niobate (LiNbO.sub.3) crystal, which has a free surface acoustic wave propagation velocity of about 3488 meters/second. On the surface of this substrate is deposited a layer of metal, such as aluminum, forming a pattern which includes transducers and delay/reflective elements.
One transducer embodiment includes a pattern consisting of two bus bars 74 and 76 connected to the dipole antenna 70, a "launch" transducer 78 and a plurality of "tap" transducers 80. The bars 74 and 76 thus define a path of travel 82 for a surface acoustic wave which is generated by the launch transducer and propagates substantially linearly, reaching the tap transducers each in turn. The tap transducers convert the surface acoustic wave back into electrical energy which is collected and therefore summed by the bus bars 74 and 76. This electrical energy then activates the dipole antenna 70 and is converted into electromagnetic radiation for transmission as the signal S2.
The tap transducers 80 are provided at equally spaced intervals along the surface acoustic wave path 82, as shown in FIG. 6, and an informational code associated with the transponder is imparted by providing a selected number of "delay pads" 84 between the tap transducers. These delay pads, which are shown in detail in FIG. 7, are preferably made of the same material as, and deposited with, the bus bars 74, 76 and the transducers 78, 80. Each delay pad has a width sufficient to delay the propagation of the surface acoustic wave from one tap transducer 80 to the next by one quarter cycle or 90.degree. with respect to an undelayed wave at the frequency of operation (in the 915 MHz band). By providing locations for three delay pads between successive tap transducers, the phase f of the surface acoustic wave received by a tap transducer may be controlled to provide four phase possibilities, zero pads=0.degree.; one pad=90.degree.; two pads=180.degree.; and three pads=270.degree..
The phase information .phi..sub.0 (the phase of the signal picked up by the first tap transducer in line), and .phi..sub.1, .phi..sub.2, .phi..sub.N (the phases of the signals picked up by the successive tap transducers) is supplied to the combiner (summer) which, for example, comprises the bus bars 74 and 76. This phase information, which is transmitted as the signal S2 by the antenna 70, contains the informational code of the transponder. The Signal S2 also includes interrogation frequency-dependent amplitude variations as a result of the encoding elements, which typically also provide information for decoding the transponder identity.
As shown in FIG. 7, the three delay pads 84 between two tap transducers 80 are each of such a width L as to each provide a phase delay of 90.degree. in the propagation of an acoustic wave from one tap transducer to the next as compared to the phase in the absence of such a delay pad. This width L is dependent upon the material of both the substrate and the delay pad itself as well as upon the thickness of the delay pad and the wavelength of the surface acoustic wave.
The transducers are typically fabricated by an initial metallization of the substrate with a generic encoding, i.e., a set of reflectors or delay elements which may be further modified by removal of metal to yield the customized transponders. Thus, in the case of delay pads, three pads are provided between each set of transducers or taps, some of which may be later removed. Where the code space is large, the substrates may be partially encoded, for example with higher order code elements, so that only the lower order code elements need by modified in a second operation.
While a system of the type described above operates satisfactorily when the number of tap transducers does not exceed eight, the signal to noise ratio in the transponder reply signal is reduced as the number of tap transducers increases. This is because the tap transducers additionally act as launch transducers as well as partial reflectors of the surface acoustic wave so that an increase in the number of tap transducers results in a corresponding increase in spurious signals in the transponder replies. This limitation on the number of tap transducers places a limitation on the length of the informational code imparted in the transponder replies.
Spurious signals as well as insertion losses may be reduced in a passive transponder so that the informational code may be increased in size to any desired length, by providing one or more surface acoustic wave reflectors on the piezoelectric substrate in the path of travel of the surface acoustic wave, to reflect the acoustic waves back toward a transducer for reconversion into an electric signal.
Surface acoustic waves may encounter frequency selective filtering structures, partial reflectors, full reflectors, phase delay pads or electroacoustic transducing elements as they travel across the substrate, which is typically lithium niobate (LiNbO.sub.3) which has a surface acoustic wave propagation velocity of 3488 m/sec and is piezoelectric. The system may have a single acoustic path or sets of acoustic paths which arc, for example, parallel, as shown in FIG. 8A.
A wavefront produced by reflections from the leading and trailing edges of transducer fingers will be formed by the superposition of a first wave reflected from the first leading edge and successive waves reflected from successive edges and having differences in phase, with respect to the first wave, of -.lambda./4, .lambda./2, -3 .lambda./4, .lambda., etc. As may be seen, the wave components having a phase -.lambda./4, .lambda./2 and -3 .lambda./4 effect a cancellation, or at least an attenuation of the wave component reflected from the leading edge.
The interdigital fingers of the transducers may therefore be advantageously split to reduce reflections. Conventional interdigital finger transducers which are constructed to operate at a fundamental, resonant frequency of 915 MHz, have a finger width (.lambda./4) of approximately 1 micron; a size which approaches the resolution limit of certain photolithographic fabrication techniques (the selective removal of metallization by (1) exposure of photoresist through a mask and (2) subsequent etching of the metallized surface to selectively remove the metal between and outside the transducer fingers). If the fingers are split, the width of each finger (.lambda./8) for a fundamental frequency of 915 MHz would be approximately 0.5 micron. The size would require sophisticated photolithographic fabrication techniques. In order to increase the feature sizes, the transducers in the transponder are constructed with a resonant frequency f.sub.0 of 305 MHz. In this case, the width of each finger is three times larger than transducer fingers designed to operate at 915 MHz, so that the width (.lambda./8) of the split fingers is approximately 1.5 microns. This is well within the capability of typical photolithographic fabrication techniques. Although such transducers are constructed with a resonant frequency of 305 MHz, they are nevertheless driven at the interrogation frequency of approximately 915 MHz; i.e., a frequency 3f.sub.0 which is the third harmonic of 305 MHz. The energy converted by a transducer, when driven in its third harmonic 3f.sub.0 (915 MHz), is about 1/3 of the energy that would be converted if the transducer were driven at its fundamental frequency f.sub.0 (305 MHz). Accordingly, it is necessary to construct the transducers to be as efficient as possible within the constraints imposed by the system. As is well known, it is possible to increase the percentage of energy converted, from electrical energy to SAW energy and vice versa, by increasing the number of fingers in a transducer. In particular, the converted signal amplitude is increased by about 2% for each pair of transducer fingers (either conventional fingers or split fingers) so that, for 20 finger pairs for example, the amplitude of the converted signal will be about 40% of the original signal amplitude. Such an amplitude percentage would be equivalent to an energy conversion of about 16%. In other words, the energy converted will be about 8 dB down from the supplied energy.
The edge portions of the delay pads, as well as the lateral edges of the bus bars and (i.e., the edges transverse to the SAW paths of travel) are advantageously provided with two levels of serrations, which substantially reduce SAW reflections from these edges. The serrations include, for example, two superimposed "square waves" having the same pulse height but different pulse periods. For example, the pulse height for both square waves is .lambda./4, and the pulse period is .lambda./3 for one square wave and 6.lambda. for the other, where .lambda. is the SAW wavelength at 915 MHz. The first level of serrations serves to reduce reflections, while the second level serves to break up the average reflection plane.
The addition of finger pairs to the transducers therefore advantageously increases the energy coupling between electrical energy and SAW energy. However, as explained above in connection with FIGS. 1, 3A and 3B, the system according to the invention operates to excite the transducers over a range of frequencies between 905 MHz and 925 MHz. This requires the transducers to operate over a 20-25 MHz bandwidth: a requirement which imposes a constraint upon the number of transducer finger pairs because the bandwidth of a transducer is inversely proportional to its physical width. This relationship arises from the fact that the bandwidth is proportional to 1/.tau., where .tau. is the SAW propagation time from one side of the transducer to the other (the delay time across the transducer).
The transducer may be divided into several separate sections: a central section, two flanking sections and two outer sections. The central section includes interdigital transducer fingers which are alternately connected to two outer bus bars and to a central electrical conductor. This central section comprises a sufficient number of finger pairs to convert a substantial percentage of electrical energy into SAW energy and vice versa. By way of example and not limitation, there may be 12 finger pairs so that the converted amplitude is approximately 24% of the incoming signal amplitude. Flanking the central section, on both sides, are sections containing "dummy" fingers; that is, fingers which are connected to one electrode only and therefore serve neither as transducers nor reflectors. The purpose of these fingers is to increase the width of the transducer so that the outer sections will be spaced a prescribed distance, or SAW delay time, from the central section. For example, there may be 7 dummy fingers (or, more particularly, split fingers) in each of the sections. Finally, each of the outer sections of the transducer contains a single transducer finger pair which is used to shape the bandwidth of the transducer, e.g., maintain an effective bandwidth of about 25 MHz.
The transducer system preferably has an electrical impedance at the design frequency which matches the impedance of the antenna coupling, to maximize the power transfer between the antenna system and transducer. This matching is accomplished by forming series connections of transducer structures, which present as capacitive loads, to reduce impedance, as necessary, and providing heavy metal traces for the bus bars to reduce Ohmic losses. The bus bars are, for example, made approximately twice as thick as the other metallized elements on the substrate.
In practice, the metallization is deposited on the substrate surface using a two-layer photolithographic process. Two separate reticles are used in forming the photolithographic image: one reticle for the transducers, reflectors and phase pads as well as the alignment marks on the substrate, and a separate reticle for the bus bars. The process thus comprises the steps of depositing a 300 Angstrom layer of chromium and then 1000 Angstrom layer of aluminum on the substrate, followed by UV-exposure solubleizing resin spin coating, masking and etching of the bus bars, followed by deposition of 1000 Angstroms of aluminum and another UV activated resin spin coating, masking and etching to form the transducers, reflectors and phase pads as well as the alignment marks on the substrate, doubling the thickness of the bus bar structures.
Each two successive fingers of a transducer may be shorted at one or more locations between the bus bars. The shorts between successive fingers reduce energy loss due to Ohmic resistance of the fingers and render the reflector less susceptible to fabrication errors.
These various techniques and systems, described above, may advantageously be employed or combined with aspects of the present invention in known manner to achieve desired results.
The embodiment of FIG. 8A comprises a substrate 120 of piezoelectric material, such as lithium niobate, on which is deposited a pattern of metallization essentially as shown. The metallization includes two bus bars 122 and 124 for the transmission of electrical energy to four launch transducers 126, 128, 130 and 132. These launch transducers are staggered, with respect to each other, with their leading edges separated by distances X, Y and Z, respectively, as shown. The distances X and Z are identical; however, the distance Y is larger than X and Z in order to provide temporal separation of the received signals corresponding to the respective signal paths. Further metallization includes four parallel rows of delay pads 134, 136, 138 and 140 and four parallel rows of reflectors 142, 144, 146, and 148. The two rows of reflectors 144 and 146 which are closest to the transducers are called the "front rows" whereas the more distant rows 142 and 148 are called the "back rows" of the transponder. The bus bars 122 and 124 include contact pads 150 and 152, respectively, to which are connected the associated poles 154 and 156 of a dipole antenna. These two poles are connected to the contact pads by contact elements or wires 158 and 160, represented in dashed lines.
The provision of four transducers 126, 128, 130 and 132 and two rows of reflectors 142, 144, 146, and 148 on each side of the transducers results in a total of sixteen SAW pathways of different lengths and, therefore, sixteen "taps". These sixteen pathways (taps) are numbered 0, 1, 2 . . . D, E, F, as indicated by the reference number (letter) associated with the individual reflectors. Thus, pathway 0 extends from transducer 126 to reflector 0 and back again to transducer 126. Pathway 1 extends from transducer 128 to reflector 1 and back again to transducer 128. The spatial difference in length between pathway 0 and pathway 1 is twice the distance X (the offset distance between transducers 126 and 128). This results in a temporal difference of .DELTA.T in the propagation time of surface acoustic waves. Similarly, pathway 2 extends from transducer 126 to reflector 2 and back again to transducer 126. Pathway 3 extends from transducer 128 to reflector 3 and back to transducer 128. The distance X is chosen such that the temporal differences in the length of the pathway 2 with respect to that of pathway 1, and the length of the pathway 3 with respect to that of pathway 2 are also both equal to .DELTA.T. The remaining pathways 4, 5, 6, 7 . . . E, D, F are defined by the distances from the respective transducers launching the surface acoustic waves to the associated reflectors and back again. The distance Y is equal to substantially three times the distance X so that the differences in propagation times between pathway 3 and pathway 4 on one side of the device, and pathway B and pathway C on the opposite side are both equal to .DELTA.T. With one exception, all of the temporal differences, from one pathway to the next successive pathway are equal to the same .DELTA.T. The SAW device is dimensioned so that .DELTA.T nominally equals 100 nanoseconds. In order to avoid the possibility that multiple back and forth propagations along a shorter pathway (one of the pathways on the left side of the SAW device as seen in FIG. 1) appear as a single back and forth propagation along a longer pathway (on the right side of the device), the difference in propagation times along pathways 7 and 8 is made nominally equal to 150 nanoseconds.
FIG. 8B shows, for a single transducer 125, connected between bus bars 122 and 124, a set o acoustic wave paths reflecting off encoding elements and returning to the transducer 125.